Atomization based dual regime spray coating system: design and applications

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Atomization Based Dual Regime Spray Coating System: Design and Applications

by

Maxym Rukosuyev

Specialist, Simferopol State University (Ukraine), 1997 BEng, University of Victoria, 2012

A Dissertation Submitted in Partial Fulfillment of the Requirements for the Degree of

DOCTOR OF PHILOSOPHY

in the Department of Mechanical Engineering

 Maxym Rukosuyev, 2017 University of Victoria

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Supervisory Committee

Atomization Based Dual Regime Spray Coating System: Design and Applications

by

Maxym Rukosuyev

Specialist, Simferopol State University (Ukraine), 1997 BEng, University of Victoria, 2012

Supervisory Committee

Dr. Martin Byung-Guk Jun, Department of Mechanical Engineering

Supervisor

Dr. Colin Bradley, Department of Mechanical Engineering

Co-Supervisor

Dr. Zuomin Dong, Department of Mechanical Engineering

Departmental Member

Dr. Alexandre G. Brolo, Department of Chemistry

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Dissertation Summary

Supervisory Committee

Dr. Martin Byung-Guk Jun, Department of Mechanical Engineering Supervisor

Dr. Colin Bradley, Department of Mechanical Engineering Co-Supervisor

Dr. Zuomin Dong, Department of Mechanical Engineering Departmental Member

Dr. Alexandre G. Brolo, Department of Chemistry Outside Member

In modern research and industrial applications, the importance of coatings can hardly be underestimated. Coatings are used extensively in optics, biomedical instruments, cutting tools, and solar panels to name a few. The primary purpose of any coating is to alter surface properties of the base material thus adding new functionality or improving the performance of the original product. A multitude of coating techniques has evolved over the years with spray coating being one of the more widely used. Some applications require deposition of materials that are either in the form of a solution or suspension. Therefore, before or during the deposition process small droplets of the said liquid are formed and transferred onto the substrate. Since differently sized droplets have different surface impact dynamics, droplet velocity at the impact plays an important role in the way it will adhere to the surface. Most spray coating techniques do not take into account the process of droplet-surface interaction which may result in overspray, poor coating thickness control, and material waste.

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The research presented in this dissertation outlines the supporting principles, design, fabrication and testing of an innovative spray coating system that provides the ability to fine tune coating parameters, including droplet impact velocities, to provide close to optimum deposition conditions. The core of the design consist of a dual velocity nozzle unit that ensures acceptable range of droplet velocities at the surface, while keeping droplets from accelerating excessively inside the system. Early experiments showed the system’s potential to produce nanoparticle coatings with particles uniformly distributed across the substrate. In addition, pigment coating for improved 3D scanning was also performed, thereby improving the surface definition and accuracy of the scanning results. Scalability of the system also led to experiments in applying this technology to microprinting. Preliminary microprinting results illustrated the system’s flexibility and opened new research avenues in micro-coating, microprinting, and, possibly rapid prototyping. Furthermore, thanks to the highly adaptable nature of the proposed design, seamless incorporation of a torch-like device into the nozzle unit was also possible. That provided the opportunity to perform in situ thermal processing or sintering of deposited material as well as production of a nanoparticle coating in a one-step process by thermally decomposing precursor solution.

Technology developed during the research work presented in this dissertation demonstrated its ability to be adapted in a number of applications that can benefit both industry and engineering research alike. Large area coatings, nanoparticle production, micro-coating, and coatings for improved 3D scanning are just a few areas where the presented technique can already, or may, if developed further, outperform existing and

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widely accepted methods. Fine tuning of the system to a particular application, and tapping into its potential in other fields will be explored in future research.

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List of Tables

Table 5-1. Experimental parameters. ... 76 Table 5-2. Average particle size for varying conditions. ... 78

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List of Figures

Figure 2-1. Modes of the droplet-surface interaction. ... 14 Figure 2-2. Nozzle with double atomizer schematics. ... 17 Figure 2-3. 3D model of the nozzle (right) and a nozzle mounted on a CNC router (left). ... 18 Figure 2-4. Exit jet: center tube out (left), center tube flash with nozzle exit (middle), center tube inside (right). ... 19 Figure 2-5. Flow patterns of the (a) jet with the mean circular jet velocity of U =3 m/s ; (b) coaxial jet with the mean center circular jet velocity, Ui = 35 m/s. ... 22 Figure 2-6. The edge of silica coated glass slide. ... 24 Figure 2-7. EDX analysis results: spectral analysis (left), and mapping of silver particles (right). ... 24 Figure 2-8. Coated glass: red ink only (left), green ink only (middle), mixed pigments (right). ... 26 Figure 2-9. Dispersion of ink pigment particles on glass. ... 26 Figure 2-10. An example of printed line. ... 27 Figure 2-11. Coating schematic and refractive indexes of coating materials, substrate and air. ... 28 Figure 2-12. Comparison of the coated vs uncoated glass sample (left); SEM image of the coating`s cross section. ... 29 Figure 2-13. Curved surface coating: nozzle cross section (left), coating setup (right). .. 30 Figure 2-14. Transmittance spectrum comparison of coated and uncoated tube. ... 30 Figure 2-15. Handheld nozzle for improved 3D scanning: 3D model (left), and assembled nozzle (right). ... 31 Figure 2-16. A coated part: Spray coating system (left); Conventional spray canister coated (right). ... 32 Figure 2-17. Sprayed and scanned aluminum part coated with conventional (a) and proposed coating method (b). ... 33 Figure 3-1. TEM images of synthesized Ag particles... 38 Figure 3-2. XRD analysis results. ... 39

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Figure 3-3. Deposition device schematics. ... 40

Figure 3-4. 3D model of the nozzle. ... 41

Figure 3-5. Optical microscope image of coated glass sample (100X magnification). .... 43

Figure 3-6. Coated glass sample (left), and SEM image of the resultant silver coating. .. 44

Figure 3-7. X-ray spectroscopy results from coated sample showing Ag map. ... 44

Figure 3-8. Water CA measurement on glass: coated (left), uncoated (right). ... 45

Figure 3-9. Water contact angle reduction on coated vs. uncoated glass surface. ... 45

Figure 3-10. Antifogging effect on glass held above boiling water. ... 46

Figure 3-11. Comparison of light transmission of dry coated vs uncoated glass. ... 47

Figure 3-12. Comparison of light transmission of coated vs uncoated glass with condensation. ... 48

Figure 4-1. System schematics. ... 55

Figure 4-2. Images of the particle jet exiting the nozzle with variable center tube location ... 55

Figure 4-3. Gas flow schematic. ... 57

Figure 4-4. Low temperature demo on the nozzle. ... 58

Figure 4-5. Coating on glass ('flamed' left, 'cold' right). ... 60

Figure 4-6.Coating on aluminum ('flamed' left, 'cold' right). ... 60

Figure 4-7. 'Cold' sample at 5000X magnification. ... 61

Figure 4-8.'Flamed' sample at 5000X magnification. ... 61

Figure 4-9. Teflon surface before treatment (optical microscope image using 100X magnification objective lens). ... 62

Figure 4-10. Teflon surface after treatment (optical microscope image using 100X magnification objective lens). ... 62

Figure 4-11. CA measurement on base Teflon. ... 63

Figure 4-12. CA measurement on nanoparticle treated surface. ... 63

Figure 4-13. CA comparison of “hot” and “cold” coated glass samples. ... 64

Figure 4-14. Light transmission for coated and uncoated glass samples. ... 65

Figure 5-1. Dual velocity nozzle with added fuel injection. ... 72

Figure 5-2. Overall system schematics. ... 74

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Figure 5-4. Particle size distribution for sample #3. ... 79

Figure 5-5. SEM image of sample #3 (concentration 200mg/l, frequency 3MHz). ... 80

Figure 5-6. SEM image of sample #6 (concentration 200mg/l, frequency 2.4MHz). ... 80

Figure 5-7: EDS analysis results ... 81

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List of symbols

Dm Mean droplet diameter.

T Liquid surface tension.

fa Ultrasonic atomiser piezo crystal resonant frequency .

We Weber number (dimensionless). Signifies relative importance of fluid inertia compared to surface tension (capillary forces).

w0 Droplet velocity normal to the surface.

ρ liquid density.

Oh Ohnesorge number (dimensionless). Relates viscous forces to inertial and surface tension forces.

Km Dimensionless number that binds together Weber number and Ohnesorge

number in order to mark the cut-off condition for splashing regime upon droplet impact onto hard surface.

Kmc Critical Km. For Km>Kmc, droplet will enter splashing regime.

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ACKNOWLEDGEMENTS

I would like express my gratitude for help and continuous support of my supervisors Dr. Martin Jun and Dr. Colin Bradley during the time of my graduate studies. Their thoughtful guidance and advice helped me to successfully complete my work and hone my skills as an engineer.

Also, I would like to acknowledge financial support from Natural Sciences and Engineering Research Council of Canada (NSERC), Korea Institute of Machinery and Materials (KIMM), and University of Victoria during the course of PhD program.

Last, but by no means least, I would like to thank my wife and best friend Olga for her love, support, understanding, and encouragement throughout the course of my

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Introduction

Chapter 1

In modern research and industrial applications, the importance of coatings can be hardly underestimated. Coatings are used extensively in optics, biomedical applications, cutting tools, and solar panels to name a few [1-6]. The primary purpose of any coating is to alter the surface properties of the base material thus adding new functionality or improving the performance of original part.

A plethora of coating methods has been developed over the years. Material deposition methods such as dip coating or spin coating [7-10] can provide adequate results in some applications where coating thickness does not play a significant role. Spin coating can be successfully used on smaller samples in cases of coating material being high viscosity liquid which does not easily lend itself for other coting methods. Dip coating can be used for large area coating in cases when coated material can be rolled into long sheets, but is difficult for larger sized samples of rigid materials. It also requires large coating facilities with significant fluid volumes. Methods such as chemical of physical vapour deposition [11-13] provide great results with outstanding thickness control (capable of depositing atomic layers of material). However, the use of these methods is limited by the necessity of having to perform the coating process in vacuum. Therefore, the areas that can be coated are usually quite small. In addition, these methods require the use of highly specialized and expensive equipment. Electroplating and anodizing are two methods that have been used extensively for metal coating [14-18]. Their use however, is limited by the properties of the substrate i.e. it should be conductive, or made conductive by prior coating in conductive materials. Plasma coating [19-21] can be successfully used on metal substrates that can withstand higher temperatures. It provides great adhesion to the

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base material but layer thickness control and thermal load exerted on the substrate during the coating process limit its use. All of the coating methods described above have their merits in particular industrial or research applications. There is, however, no such thing as a universal coating technique that can be used with any material on any substrate. Further review of coating methods relevant to the research presented in this dissertation can be found in each respective chapter.

Spray coating is one of the more widely used methods of depositing materials. Most spray coating techniques however, do not take into account the process of droplet-surface interaction which may result in overspray, poor coating thickness control, and material waste. Research presented in this dissertation is aimed at developing an efficient and cost effective novel spray coating technique which incorporates droplet velocity control based on droplet size. Furthermore, testing the system in real world engineering applications formed another part of the research.

The basic idea to apply droplet dynamics control to coating processes stemmed from the previous research in the use of spray coolant/lubricant for milling application [22] and the later study of its effects in micro-milling [23, 24]. Specific requirements for the coolant/lubricant application in micro-milling led to the concept of cutting zone penetration in order to provide appropriate temperature and lubricity control during the cutting process. Since flood cooling is next to impossible in micro-milling due to the very high rotational speed of the tool (which creates cavitation and prevents efficient cooling of the cutting zone), forced spray cooling was proposed as a novel approach to cooling in micro-machining. To ensure efficient cutting zone penetration and spreading of the coolant droplets, the spray should be delivered with velocities that fall in a certain range.

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Early experiments in micro-milling cooling systems produced basic concepts that later were used in the coating system development. However, this technology could not be directly applied for spray coating. Nozzles with special geometry had to be developed to effectively deliver atomized droplets of material to the coated surface.

The novelty of the presented work is primarily found in the development of the coating nozzles that create a dual velocity regime inside the coating nozzle. As a result of droplets having different velocities inside and outside of the nozzle, condensation inside the nozzle can be avoided whereas the spreading of the droplet on the coated surface is ensured. Furthermore, once applied onto the surface, many nanoparticle coatings require post curing or sintering to provide better adhesion and/or remove polymer nanoparticle encapsulation [25]. Furthermore, some coatings use pyrolysis to facilitate the formation of nanoparticles from a precursor solution [26]. However, injection of the particles into the flame from the side produces uneven heating of the particle stream which leads to lower deposition rates, more waste, and reduced coating quality. Using concentric geometry intrinsic to the proposed nozzle design, a thermal spray treatment setup could be seamlessly incorporated into the device. Therefore, the resulting coating system can be used both in “cold” (for room temperature coating), and in “hot” (when elevated temperatures are required) mode of operation. “Hot” operational conditions provide the means to either sinter/heat-treat particles already contained within spray droplets, or generate new particles using pyrolysis of the precursor solution. The latter can be used either for nanoparticle production or as a one-step coating process that eliminates the need for a costly separate particle production stage.

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The technology presented in this research forms a basis for further development of the dual velocity coating system. Further tailoring of this technique to a particular application’s requirements is needed to achieve optimum results. Some examples of the system’s capabilities in optical coatings, hydrophilic coatings, nanoparticle generation, and coating for 3D scanning are presented in later chapters of this dissertation.

Dissertation outline 1.1

This dissertation consists of four papers that have already been published, submitted for publication to a peer-reviewed academic journal, and/or presented at international conference. Information on each publication is located at the beginning of each chapter.

Chapter 2 discusses the need for the development of a novel coating device that is able to overcome the shortcomings of the coating systems that are presently available. Furthermore, the chapter outlines the theory behind droplet impact dynamics, which serves as a foundation for the design of the device, and the fluid flow parameters that have to be achieved in order to produce desired coating results. Next, the design idea of a dual velocity coating system with a separate ultrasonic atomization unit is introduced and its functionality explained. Chapter 2 concludes with the preliminary evaluation of coating results obtained by using the system. Such applications as microprinting, antireflective coating on glass surfaces, and coatings for improved 3D scanning are also explored and relevant coating results are presented at the end of the chapter.

Chapter 3 focuses on the application of the technology described in Chapter 2 for applying antireflective and antifogging silver nano-particle coatings onto a glass substrate. Silver is used extensively in medical applications for its antimicrobial/antibacterial properties. In addition, silver particles are utilized in coating of photovoltaic panels to

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reduce fouling and increase efficiency. The combination of material properties, particle size and uniform application of the nanoparticles produces a superhydrophilic surface, which in its extreme also has antifogging properties. The antifogging effect is created when the liquid contact angle on the surface is reduced rapidly thereby eliminating light scattering off of the individual droplet surfaces. The antireflective properties of the obtained coating can also be combined with the antibacterial effect found on optical devices to produce surfaces that provide less grip for bacteria or mold, thus having antifouling properties.

Chapter 4 is a continuation of the previous chapter as the experiments are carried out using the same silver nanoparticles as coating material. The coating method however, is altered by adding a torch-like device which facilitates in situ thermal treatment of nanoparticles. Heating nanoparticles can serve several functions such as rapid solvent evaporation, disposing of an encapsulating polymer left after particle production, sintering, and simultaneous heating of the substrate for better adhesion. The ability of the system to effectively process nanoparticles at elevated temperatures and deposit them onto the substrate surface was successfully demonstrated.

Chapter 5 describes the use of the system for a one-step coating process that includes production of the nanoparticles and their subsequent deposition onto the substrate. Using the same approach as in the previous chapter of creating a “hot” region on the droplet’s path of travel, a solution of silver nitrate is instead of silver nanoparticle suspension. At elevated temperatures, silver nitrate decomposes producing silver nanoparticles as a result of the pyrolysis process. As the droplet travels through the flame, silver particles are formed and deposited onto the substrate. This process eliminates the need for an

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additional step of particle production and can be used with other precursor solutions to produce coatings with different functionality.

Research contributions 1.2

Research presented in this dissertation represents a gradual development of the idea of coating velocity control. Room temperature operation optimization was the first milestone that was reached with the development of the dual regime coating system. Further “probing” of the system’s potential, revealed its great adaptability and ease of modification to expand its functionality. Downscaling of the system led to early experiments in micro-printing. Furthermore, the addition of the heat treatment arrangement allowed utilization of system’s added functionality by potentially eliminating or significantly reducing post-curing of the resultant nanoparticle coating. Flame spray pyrolysis technique developed using annular flame design moved the efficiency of the system yet further by allowing the in situ synthesis of nanoparticles for simultaneous coating or collection for further processing.

The novel contributions that result from this research work and are presented in this dissertation are summarized below:

i) Development of a novel coating device: A coating device that uses dual velocity

nozzle is designed developed and initially tested. The nozzle creates low velocity flow internally to prevent condensation and formation of larger droplets while providing close to optimum velocity at the surface. The nozzle provides the means to conduct additional sorting of droplets by size, which creates a more consistent coating and results with minimal or no overspray or wasted material. Thanks to its intrinsic scalability, the system can be easily adapted to large area

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coatings as well as to micro-printing with only minor design modifications. Since the droplets are produced separately from the deposition unit, the user can utilize several droplet sources with various materials and mix them just prior to deposition allowing for production of composite coatings (see Chapter 2). This work was published in the Journal of Coatings Technology and Research [27].

ii) Antifogging/superhydrophilic and antireflective silver nanoparticle coating:

As the coating system introduced in Chapter 2 was capable of producing sub-micron thickness coated layers, silver nanoparticles synthesized “in house” were used to create functional glass coatings. As a result, fine, nanometer scale structure was created on the surface that exhibited strong superhydrophilic behaviour and produced antifogging effect. In addition, the ability of the resultant coating to increase light transmission in the visible spectrum was also discovered. That unique combination, together with silver’s antibacterial properties, creates a coating that can potentially be attractive in several industrial and research applications for biomedical devices, optics, photovoltaic panels, etc. (see Chapter 3). This paper was published Journal of Coatings Technology and Research [28].

iii) Thermally assisted silver nanoparticle coating: Based on the same general

nozzle design introduced in Chapter 2, a new feature was added to facilitate in-process thermal treatment of the droplets/particles. Since the material used for experiments is similar, resulting coating exhibits properties like those seen in Chapter 3. However, the use of a built in torch-like device enables removal of any polymer remaining on the surface of particles after they have been produced. Due to its unique concentric design, the nozzle introduced in this chapter will produce

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a “hot” region that has a dome-like shape and is completely axisymmetrical with the droplet jet. This unique feature eliminates the negative effect of thermophoresis that can induce particle migration along the thermal gradient direction from “hot” to “cold” regions. Therefore, more droplets/particles are heated evenly thus undergoing the same thermal treatment producing more consistent coating results (see Chapter 4). This paper was presented at the 11th International Conference on Micro Manufacturing (ICOMM 2016) and published in ASME Journal of Micro- and Nano-Manufacturing [29].

iv) Novel one-step coating process with in situ silver nanoparticles synthesis: The

research presented in Chapter 4 builds on the previous chapter and extends the concept of the thermal treatment of atomized droplets. As opposed to the experiments described in Chapter 3, elevated temperature at the exit of the nozzle in this case facilitate pyrolysis process. Droplets of the precursor solution, while passing through the “hot” portion of the nozzle, are heated to the level where thermal decomposition of the precursor can occur. Nanoparticles are formed as a result of this chemical reaction. Factors like the size of a droplet, travel time of the droplet through flame, precursor concentration, and maximum flame temperature play an important role in the formation of particles. The system presented in this paper allows for tuning of the parameters to achieve desirable particle characteristics as well as simultaneous deposition of formed nanoparticles onto a substrate (see Chapter 5). This paper was presented at the World Congress on Micro and Nano Manufacturing (WCMNM 2017) and its extended version (in

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this dissertation) is submitted for publication to the Journal of Coatings Technology and Research.

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Design and application of nanoparticle coating system with

Chapter 2 decoupled spray generation and deposition control

This paper was published in Journal of Coatings Technology and Research in 2016. Maxym Rukosuyev, Oleksandr Barannyk, Peter Oshkai, and Martin Jun.” Design and

application of nanoparticle coating system with decoupled spray generation and deposition control”, Journal of Coatings Technology and Research, 13(5) (2016), pp.

769-779.

This chapter introduces problems faced by the spray coating in general and outlines possible solutions. Operating principles, design, and parameters tuning for the novel spray coating technique are described and evaluated. In addition, a series of experiments illustrating system’s ability to deliver cost effective solution in various coating applications is also presented.

Contributors:

Maxym Rukosuyev – designed, manufactured, tested the coating system, and

completed the manuscript.

Oleksandr Barannyk – aided in the setup and data processing during the PIV analysis,

and compiling the corresponding section of the manuscript.

ABSTRACT

Coatings are widely used in various applications to change the interaction of the surfaces with external media. The key factors that determine the quality of a spray-coated layer are the size (order of a few microns in diameter) and dimensional uniformity of droplets in the spray and the droplet impact velocity. For many applications, coating quality is strongly dependant on the method and equipment used during the application process. This paper presents development of a decoupled system for spray coating and micro printing, which includes an ultrasonic spray generation device and a nozzle for the spray deposition independently operated. Design and development of the system as well

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as testing for different applications are presented in this paper. The system design can be potentially used for large area coating, such as windows and solar panels, as well as micro printing of electronic circuits and numerous other applications.

Introduction 2.1

The range of applications requiring various types of coatings stretches from biomedical devices to microelectronics, and the number of technological solutions provided by the use of advanced coating materials is still growing. Coatings are used extensively to enhance the surface properties of different substrates. Such properties include abrasion resistant [6] and antireflective coatings[5, 30] on glass surfaces in various devices or wear resistant coatings[1] on friction surfaces of various mechanical components. In addition, conductive, light sensitive, and conformal coatings are frequently used in the construction of printed microelectronic devices and photovoltaic panels [4, 31]. Coatings are widely used in various biomedical applications to change the interaction of the surfaces with bioactive materials[2]. Two characteristics of the coatings that are of primary importance are the coating uniformity and coating thickness control. The key factors that determine the quality of a spray-coated layer are the size (order of a few microns in diameter) and dimensional uniformity of droplets in the spray and the droplet impact velocity[32]. For many applications, coating quality is strongly dependant on the method and equipment used during the application process.

Quite often, to produce a functional coating, suspensions of various nanoparticles are used. In that case, atomised droplets will contain nanoparticles within, which will be left on the coated surface once the liquid solvent is evaporated. The need for small size of the

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atomized droplets arises from the fact that in order for the nanoparticle coating to be uniform across the surface, one would require the particles to be finely dispersed within an atomised droplet. In the case of larger droplets, nanoparticles inside the liquid tend to agglomerate[33]. Therefore, once deposited, nanoparticle agglomerates are left on the surface thus increasing surfaces’ roughness and decreasing the coating’s uniformity. Consequently, the fewer nanoparticles are in a single droplet, the more uniformly they will be distributed over the coated surface and the better the coating thickness control will be.

Traditionally, conventional air pressure atomising nozzles are used for spray coating [34-36]. Although providing adequate performance in most spray coating applications, drawbacks associated with this method make it unsuitable when tight thickness and uniformity requirements are in place. Due to the nature of the atomisation mechanism in air atomising nozzles, one should have certain air pressure to ensure atomisation. That limits the range of spray velocities that can be controlled. In addition, droplet size distribution is non-uniform, strongly dependant on the pressure applied, and generally in the region of tens to hundreds of µm.

One way to produce droplets of consistently small size is to use an ultrasonic atomization process [37-39]. Atomization of the coating solution/suspension with the aid of a vibrating piezoelectric crystal provides the spray droplets of desired size and can accommodate the use of wide range of materials. Several companies, such as Sono-Tek, Sonaer, and Optomec are successfully using ultrasonic atomization processes to generate aerosols used for coatings and micro printing (in the case of Optomec). However, Sono-Tek and Sonaer spray nozzles can only be used for low frequency vibration (up to 130

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kHz), which limits the diameter of the sprayed particles to a minimum of ~12 μm (in the case of water as a solvent). Furthermore, the above mentioned nozzles may require additional mechanism of spray acceleration to optimize the particle velocity at the impact. Optomec's micro printing equipment atomizes the solution in a separate chamber (hence higher frequency of vibration and smaller particle size) and uses so called "shroud air" to accelerate the particles pushed through a small diameter (order of ~100 μm) channel, which might cause clogging problems. Additionally, the Optomec system is designed primarily for microprinting and not intended to be used for large area coating.

The objective of this paper was to develop an innovative system for the spray coating and micro printing which will include an ultrasonic particle generation device and a nozzle for the spray deposition. The deposition nozzle, due to the larger diameter of the outlet channel, allows for a high throughput of droplets and simultaneously will accelerate and focus spray droplets using central high-speed air flow in the center of the nozzle exit. Therefore, the proposed system attempts to solve both, droplet size and uniformity and proper droplet impact velocity problems to achieve close to optimum coating conditions. The described design could be successfully used for large area coating, such as windows and solar panels, as well as micro printing of electronic circuits and numerous other applications.

Note on particle impact dynamics 2.2

To provide an optimum regime for the liquid particle delivery and application, we need to consider its size and velocity. The main parameters that determine the size of atomized particles are the frequency of the ultrasonic nebulizer, liquid density, and liquid

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surface tension. The empirical formula that gives a fair estimate of the mean particle diameter is[40],

𝐷_𝑚 = 0.34 ∗ √8𝜋 ∗ 𝑇 𝜌 ∗ 𝑓_𝑎2

3

where Dm is droplet diameter, T is liquid surface tension, ρ is liquid density, and fa is

nebulizer frequency. Knowing the mean droplet size for a given nebulizer frequency, we can determine the corresponding desired velocity of the droplet, which will govern the droplet-surface interaction.

In general, there are three possible regimes of liquid droplet-solid surface interaction: spreading, rebounding, and splashing (see Figure 2-1). For a coating system consisting of a spray generation unit and an application nozzle, it is important to establish a rebounding regime within the system before the droplets exit the nozzle [40]. That will minimize the condensation of liquid on tubing/nozzle walls and unnecessary buildup of material on the internal surfaces.

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To ensure a stick condition or to control the particle impingement, a set of non-dimensional numbers is used. For the particle to rebound from the surface upon the impact, Weber number (We) provides the transition condition[41]:

𝑊𝑒 = 𝑤02𝐷𝑚𝜌 𝑇

where Dm is mean particle diameter, T is liquid surface tension, ρ is liquid density, and

w0 is velocity normal to the surface. In order to provide rebounding regime for the

travelling droplets to the nozzle, we must ensure that the condition We<10 is satisfied. On the other hand, droplets on the target surface should neither rebound nor enter the splash regime (when the droplet will disintegrate into a number of smaller particles upon impact). To provide optimum distribution and adhesion of deposited material, droplets should spread on the surface forming a thin film. Therefore, another characteristic dimensionless number (Km) is used to provide a guideline in the system design process.

To prevent splashing, the condition Km<Kmc should be satisfied (where Kmc = 57.7)[32].

The formula used to compute Km is

𝐾𝑚 = (𝑂ℎ−2 5⁄ ∗ 𝑊𝑒) 5 8 ⁄ where 𝑂_ℎ = 𝜇 √𝑇 ∗ 𝜌 ∗ 𝐷𝑚

where Oh is Ohnesorge number and µ is liquid dynamic viscosity.

Therefore, to satisfy the rebound condition on the nozzle’s internal surfaces as well as prevent splashing on the target surface, a dual-regime nozzle should be used. While a low velocity rebound regime is desirable on the internal surfaces of the device, droplets have to have higher velocities at the substrate surface to ensure a splashing regime to achieve

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efficient deposition with minimal material waste. Moreover, by decoupling the particle generation device and the application nozzle, compared to pressure-based devices, much better control of the spray velocity can be achieved, which will be explained more in detail in the next section.

Design 2.3

2.3.1 Design concept and system development

The core of the design consists of the atomized droplet generation and the deposition nozzle decoupled in their respective operation. The low velocity carrier gas takes the droplets from the droplet generation unit to the deposition nozzle. At the nozzle exit, the particles are accelerated and focused by the high speed gas flow from the tube in the center of the nozzle. Therefore, a dual velocity regime is realized by using the lower velocity carrier gas internally, while the droplets are accelerated at the nozzle exit just before the deposition occurs. To avoid condensation, facilitate droplet size uniformity, and ensure symmetry of the particle stream, an extra intermediate flow-conditioning unit (FCU) was added to the design. FCU also doubles as a mixing chamber when two or more materials are deposited simultaneously (Figure 2-2).

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Figure 2-2. Nozzle with double atomizer schematics.

The FCU is placed in between the atomizer(s) and the nozzle and consists of a hollow cylinder with honeycomb structure imbedded in it. The asymmetric entrance ensures a vortex forming in the lower section to sort the particles. Due to a ‘cyclone’ effect, larger particles will collide with the wall of the chamber, condense, and will be drained out of the FCU. Smaller and lighter droplets will travel through the honeycomb and further down to the nozzle exit. The carrier gas stream is also made less turbulent while passing through the honeycomb structure. Furthermore, the FCU allows the addition of the long high-speed central gas tube, which creates an axisymmetric exit channel thus reducing turbulent disturbances within the nozzle and at the nozzle exit. A 3D model of the nozzle design and the fabricated nozzle mounted onto a CNC router (Romax CNC, USA) are shown in Figure 2-3.

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Figure 2-3. 3D model of the nozzle (right) and a nozzle mounted on a CNC router (left). The nebulizer used was based on the APC International atomizer board with 1.65MHz piezoelectric element, rated input voltage of 48V, and mist output ratio of ~350cc/hr. The mist flow rate was set at 0.1-0.15 m3/min to satisfy low velocity condition within the nozzle. The average droplet size was measured to be 6.0 µm with 1.65 MHz[42].

The exit jets of atomized droplets from the nozzle at different conditions are shown in Figure 2-4. The form of the exit jet is dependent, among other things, on the position of the high-speed center tube with respect to the nozzle exit. The “dome” at the nozzle exit is actually a stable vortical structure, as can be seen from high-speed imaging and described in more details in a later section. Figure 2-4 clearly shows that most of the atomized droplets are tightly focused into a high-speed jet. Consequently, the accelerated particles are deposited onto the substrate in such a way as to maintain close to optimum coating conditions.

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Figure 2-4. Exit jet: center tube out (left), center tube flash with nozzle exit (middle), center tube inside (right).

In order to produce a coating with droplets uniformly distributed over the substrate’s surface, a series of experiments were conducted to determine close to optimal distance of the nozzle tip from the substrate. For a nozzle with an exit diameter of 15 mm, the distance was determined to be approximately 30-35 mm. As the nozzle was mounted on a CNC router platform, the feed rate (i.e. the lateral velocity of the nozzle motion across the surface) was also optimized. The combination of appropriate nozzle exit distance and feed rate produced a coating layer where the solvent was evaporated within 1-2 seconds after deposition. This rapid evaporation ensured that smaller droplets would not coalesce, thus providing the opportunity for the nanoparticles to settle on the surface before they could agglomerate into large clusters.

2.3.2 High speed imaging and PIV analysis of the resultant jet

To study the overall structure of the jet forming at the nozzle exit, a series of images using high speed recording equipment and PIV technique were analyzed. The near-field structure of a coaxial jet is considerably complex. The mixing between the jet streams is critically controlled by the dynamics and interactions of the vortical structures in

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shear-layers developed between the two jets, and also between the outer jet and the ambient fluid. To study the effects of controlled perturbation, the inner jet was turned off and on, as illustrated in Figure 2-5 (a) and (b), respectively. In both cases, the jet propagated freely into the unbounded area until it impinged on a 3 mm plate, located 40.5 mm downstream. The images shown in Figure 2-5 were collected using particle image velocimetry (PIV). The PIV system, implemented to acquire quantitative images of the air flow across the cavity opening, consisted of an Nd:YLF dual cavity laser (power output of 22.5 mJ/pulse at 1 kHz, wavelength of 527 nm), which provided illumination of the flow field. The flow was seeded with Di-Ethyl-Hexyl-Sebacat (DEHS) droplets with a mean diameter of 1 µm. The particles were generated using a flow seeder that employed Laskin nozzles (model # 1108926, LaVision GmbH, Germany), which inject high velocity streams of compressed air into an oil bath. The air jets atomize the oil and produce a polydisperse oil aerosol. The particle concentration was controlled by employing a secondary air inlet to the atomizer, which contained a flow rate regulator. The Stokes number of the droplets (St = ρpdp2U/18μL, where ρp and dp are density and the

diameter of the tracer droplets respectively, μ is the dynamic viscosity of the fluid, U is the droplet velocity, and L is a characteristic length) was equal to 2.8×10-3, which indicated that the droplets were sufficiently small to accurately follow the flow. The images of the tracer droplets were recorded with a digital Complementary Metal Oxide Semiconductor (CMOS) camera equipped with a sensor that consisted of 1024 × 1024 pixels.

Initially, the center gas was turned off and the flow was allowed only through the outer region of the nozzle. As a result, the impinging jet formed for which the structure

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can be visually separated into three main regions: R1 - Free jet region, R2 - Stagnation region, and R3 - Wall-jet region. When the center gas is not present, the large vortical structure, represented by a primary vortex, was periodically formed and shedded from the nozzle, as it can be seen in Figure 2-5(a). Spectral analysis of the temporal modes of the jet, obtained through proper orthogonal decomposition (POD), showed that the shedding frequency of these large vortical structures was about 137 Hz. As the flow is deflected from the impinging wall, a wall jet was developed. The wall jet separated due to the interaction with the primary vortices and the impinging wall, and as a result, secondary vortices, with shedding frequency of 235 Hz were formed. The interaction of the primary vortices with the wall shear layer gave rise to unsteady vortical motions. Additionally, the presence of the large vortical structures created a rather large impinging area of the jet, with the diameter larger than that of the nozzle.

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Figure 2-5. Flow patterns of the (a) jet with the mean circular jet velocity of U =3 m/s ; (b) coaxial jet with the mean center circular jet velocity, Ui = 35 m/s.

The introduction of the center jet led to the formation of a proper coaxial jet and changed the dynamics of the flow significantly. Because the vortices in the outer shear layer roll up, the vortices in the inner shear layers cannot be easily identified as separate entities. It can be seen from Figure 2-5(b) that vorticities are connected to each other, indicating that three-dimensional effects are of prime importance for the vertical structure. Spectral analysis of the temporal modes of the coaxial jet showed that the vortical

R1

R3 R2

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structures with shedding frequency of 215 Hz and 274 Hz dominated the flow field, with the lower frequency corresponds to the outer jet and the higher frequency corresponds to the center or inner jet, respectively. The presence of the inner jet led to a substantial decrease of the all three regions of the impinging jet and allowed it to focus the coaxial jet on a much smaller area, compared to that in the previous case.

2.3.3 Uniform coating performance testing

In order to test the uniform coating distribution performance of the nozzle, a number of coating experiments were conducted. These experiments indicated that the nozzle design allows for the particles to be uniformly deposited onto a variety of substrates with different geometries.

2.3.3.1 Encapsulated silica

Polymer-coated silica nanoparticles were coated onto glass a substrate with subsequent sintering at 500oC for 4 hours. Nanoparticles used in this experiment were Vive Nano Silica (+) (by Vive Nano, Canada) polymer encapsulated silica nanoparticles (water based suspension with 0.1% of nanoparticles by weight). Results shown in Figure 2-6 indicate a uniform thickness with a very slight variation in the range of about 10 nm. The image was taken using Hitachi S-4800 field emission scanning electron microscope and QUARTZ PCI (version 8) software. The polymer used to encapsulate the particles is evaporated during the sintering process. The remaining nanoparticles form a coherent layer sufficiently covering the substrate with no observable gaps in the coating.

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Figure 2-6. The edge of silica coated glass slide.

2.3.3.2 EDX results pure silver on glass

Pure silver nanoparticles were also deposited onto a glass substrate. To highlight the distribution of the nanoparticles on the substrate, an EDX analysis has been performed (Hitachi S-4800 SEM). The resultant spectrum and the accompanying mapping image are shown in Figure 2-7. EDX results clearly show that silver nanoparticles are evenly dispersed on the glass substrate surface. The particles were synthesized in house and the average is around 20-30 nm in size. The modified polyol process was used to synthesize the nanoparticles [43].

Figure 2-7. EDX analysis results: spectral analysis (left), and mapping of silver particles (right).

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2.3.4 Dual inlet test

To verify that the dual inlet design produced a uniformly dispersed mixture of the two different nanoparticle suspensions, coatings with two acrylic inks of green and red color containing around 100 nm sized pigments were applied onto a glass substrate. The resultant coatings were examined using an optical microscope (Zeta-20 optical profiler, Zeta Instruments, USA). The red pigmented ink was first used to coat a glass substrate; then, the green pigmented ink was used to coat a different glass substrate. In order to test the on-the-fly mixing capability of the FCU unit in mixing of two sets of droplets generated by two independent atomizers, the red and green pigmented inks were atomized simultaneously by two different atomizers and the two sets of droplets were mixed at the FCU unit. Then, the mixture of droplets were sent to the nozzle for coating. The acrylic pigmented inks used were Daler Rowney FW acrylic ink in red and green and were diluted to 5% with distilled water. Comparison of the samples coated with red ink, green ink, and dual coated glass is seen in Figure 2-8. Although there are no brown color pigments used (only red and green), because of the presence of red and green pigments uniformly distributed on the substrate, the coated mixed pigments show brown color. This confirms the uniform on-the-fly mixing capability of the FCU unit and indicates simultaneous deposition possibility of foreign nanoparticles that might not mix well in solution.

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Figure 2-8. Coated glass: red ink only (left), green ink only (middle), mixed pigments (right).

A closer look at the coating uniformity using optical microscope reveals that the pigment nanoparticles are evenly dispersed over the coated surface (see Figure 2-9). These results also confirm the assumption that the small droplets containing ink pigment particles will evaporate during the coating process before they would have the opportunity to form larger droplets on the coated surface.

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Applications and Results 2.4

Due to the system’s inherent adaptability, the number of possible applications is quite large as various coatings are used extensively throughout industry and in many research areas. Some of the possible applications the system was adapted to be used in are illustrated in subsequent sections.

2.4.1 Printing

First attempts in using the initial design of the system were done in micro-printing. Because the atomization function was removed from the nozzle design, it is relatively much easier to downscale the design for micro-printing. Figure 2-10 shows printed lines of ~80 μm in width, printed using a miniaturized nozzle. Although the system was not specifically designed for micro-printing but was only downscaled, the results clearly showed its potential to be used in micro-printing.

Figure 2-10. An example of printed line.

2.4.2 Antireflective coating

The coating system was also used for antireflective (AR) coating applications. Antireflective (AR) coating applied with the coating system showed significant

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improvements as far as the light transmission and reflection spectrum is concerned. AR coating consisted of subsequent layers of TiO2 and SiO2 films of approximately 100 nm

thickness each. Figure 2-11 illustrates a schematic of how these films are applied to achieve an AR function as well as the refractive index values of each material. The coated surface was subsequently sintered at 550 0C for 4 hours to remove the polymers encapsulating the nanoparticles. Subsequently, the transmittance spectrum of the coated samples was measured using an Ocean Optics USB4000 spectrophotometer equipped with a DT-MINI2 light source. The AR coating results are shown in Figure 2-12. Evidently, words behind the glass slide can be seen much better through the coated area, whereas, due to light reflection, it is not as easy to see the words through the uncoated area. Figure 2-12 (right) also shows the uniformity of the coating thickness including both TiO2 and SiO2 coatings; only a few nm variations can be observed in the SEM

image of the coating. The surface roughness values (Ra) of the resultant coating were measured to be between 0.035 and 0.04, compared to the gold sputtered surface roughness of ~0.02.

Figure 2-11. Coating schematic and refractive indexes of coating materials, substrate and air.

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Figure 2-12. Comparison of the coated vs uncoated glass sample (left); SEM image of the coating`s cross section.

Furthermore, the coating system proved to be highly adaptable to varying surface geometry such as the interior surface of a glass tube. Figure 2-13(right) depicts the coating system used in AR coating of the inner surface of a glass tube of 1.5” diameter. A horizontal coating nozzle was designed and fabricated for this purpose as shown in Figure 2-13 (left) in order to be able to coat the tube interior surface. The coating process involved simultaneous motion of the nozzle along Z-axis at 4.0 in/min and tube rotation at close to 60 RPM. The tip of the nozzle is situated approximately 5 mm from the inner tube surface. Similar to flat glass samples, the tube was heated to 5500C for 4 hours to remove the polymer encapsulating the nanoparticles. The resulting transmittance spectrum through the wall of the tube is shown in Figure 2-14. As shown, light transmission is improved over the coated tube due to the AR coating.

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Figure 2-13. Curved surface coating: nozzle cross section (left), coating setup (right).

Figure 2-14. Transmittance spectrum comparison of coated and uncoated tube.

2.4.3 Coating for Improved 3D Scanning

Optical 3D scanning techniques are widely used in industry, research, archeology, and other fields for creation of digital solid models of physical objects. Some optical 3D scanning techniques, however, can struggle to obtain accurate representation of surfaces because optical scanning is difficult over shiny or black surfaces. In most cases, the object to be scanned is coated with materials of better optical property using a spray can

z x

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or spray gun. However, conventional spray coating techniques often cause over-spraying, excessive layer thickness, and “blind” spots in difficult to reach areas.

Because coating for optical 3D scanning is typically done manually, a hand-held nozzle was designed for use with atomization coating system as can be seen in Figure 2-15. This nozzle was used to improve the ability of the scanner to create an accurate 3D surface by applying a thin coat of opaque paint. The hand held nozzle has a trigger valve that activates a high speed gas jet to focus and accelerate the spray supplied from the atomizer. Therefore, it can be used in much the same way as a conventional spray gun.

Figure 2-15. Handheld nozzle for improved 3D scanning: 3D model (left), and assembled nozzle (right).

A comparison of the effectiveness of the proposed system with the conventional spray, showed that the developed nozzle configuration is capable of producing scanning results that more accurately represent the actual physical object. A machined aluminum part was coated with the proposed system as well as using a conventional method utilized in 3D scanning as shown in Figure 2-16. After being sprayed, the part was scanned using a HDI Blitz structured-light 3D Scanner with accuracy of approximately 120 µm. As is

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commonly done, several scanning passes resulted in an image of the coated surface comprised of a point cloud. The scanning results are shown in Figure 2-17. As can be observed, the corner that was coated with the proposed system matched the actual geometry of the part much better than the conventionally coated one. Also, some air pockets can be seen over the conventionally coated corner area. This result can be explained by a significantly thinner layer of coating on the surface, hence a minimal change to the part geometry resulting in a more accurate representation of the actual part. The coating thickness on the flat portion of the part was recorded as ~10 µm for conventional coating versus ~0.2-0.4 µm for the proposed coating system. The surface roughness (Ra) recorded for the conventionally coated sample was approximately 0.3, whereas the coating applied with the system gave a much lower value of ~0.04.

Figure 2-16. A coated part: Spray coating system (left); Conventional spray canister coated (right).

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Figure 2-17. Sprayed and scanned aluminum part coated with conventional (a) and proposed coating method (b).

Conclusion 2.5

In this manuscript a coating system is presented and its performance is tested and analysed for different coating applications. The core principle of the system’s operation is the decoupling of the droplet generation device and the nozzle that facilitates close to optimum coating conditions. A dual regime nozzle ensures the particles travel through the nozzle at relatively low velocity to prevent particle adhesion to the interior walls. At the same time, at the exit of the nozzle the particles are accelerated to enter the “spreading” regime as outlined in the particle impact dynamics section. The unique tunability and adaptability of the system configuration allows nozzle design changes suitable for a number of different coating applications. The potential of the system to be used in conventional spray coating and micro printing have been demonstrated.

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Uniform silver nanoparticles coating using dual regime

Chapter 3 spray deposition system for superhydrophilic and antifogging

applications

This paper was published in Journal of Coatings Technology and Research in 2017. Maxym Rukosuyev, Ahmad Esmaeilirad, Syed Baqar, and Martin Jun.” Uniform

silver nanoparticles coating using dual regime spray deposition system for superhydrophilic and antifogging applications”, Journal of Coatings Technology and

Research, 14(2), (2017), pp. 347-354.

This chapter describes the process of using dual velocity deposition nozzle to produce uniform sub-micron thick layer of silver nanoparticles to create antifogging and antireflective effect on glass surface. The scope of this chapter include updated nozzle design and its potential application in coatings on optical devices.

Contributors:

Maxym Rukosuyev – designed, manufactured, tested the coating system, and

completed the manuscript.

Ahmad Esmaeilirad – synthesized materials (silver nanoparticles) used in coating

experiments and helped in compiling the corresponding section of the manuscript.

Syed Baqar – helped in conducting the experiments, data recording, and analysis.

ABSTRACT

Antifogging and/or superhydrophilic properties of coatings are widely exploited both in a lab environment and in industrial applications. Material choice for the production of the said coatings is of vital importance for the final coating properties. Silver nanoparticles have been known to have antibacterial and fungicidal properties, which make them extremely useful in biomedical applications. Production of coatings that combine superhydrophilic and the unique properties of silver nanoparticles can be beneficial in numerous applications. Dual regime spray coating system allows the

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production of thin, uniformly distributed nanoparticle coating through droplet impact velocity and gas flow control. Silver nanoparticles of ~15nm average diameter were synthesized and coated onto the glass substrate, which produced the top layer with strong superhydrophilic/antifogging properties with water contact angles of close to 6 degrees. In addition, coated surfaces exhibited an increase in light transmission of ~0.7% in 500nm-700nm range.

Introduction 3.1

Superhydrophilic coatings are used extensively both in industry and active research. The applications for the surfaces exhibiting superhydrophilic properties include anti-fogging, drag reduction, enhanced heat transfer, anti-fouling, anti-reflection, biologically clean surfaces, and self-cleaning surfaces [6, 44, 45]. Some specific applications in biomedical field include the use of superhydrophilic surfaces to enhance antifogging properties of medical imaging devices [46, 47], increased resistance against absorption of proteins, bacteria, and fibroblast cells [48-50], reduce thrombosis [51], and improve bone healing and osteogenesis [52] among others. However, the production of superhydrophilic surfaces, in some cases, requires the use of a specific stimuli to trigger superhydrophilisity. Titanium dioxide and zinc oxide coated surfaces, for instance, can produce superhydrophilic effect when activated by UV light [44, 45, 53] and will lose their properties once being unlit for a prolonged period of time. Other stimuli include the use of mechanical stress, electric potential, and change in temperature to name a few [45, 54-57].

Silver nanoparticles have several attractive properties that make them useful in a number of applications. The use of silver nanoparticles for their antibacterial [58-60] and

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antifungal [61-63] properties is widely accepted and described in literature. Incorporating silver nanoparticles into SiO2 coatings [64] or using it directly on the Si substrate [65]

produces antireflective effect which can significantly increase the efficiency of electro-voltaic solar cells. The unique combination of bactericidal, fungicidal, and superhydrophilic behaviour of the obtained coating may prove to be a considerable benefit in a number of biomedical applications. Furthermore, superhydrophilic properties of the obtained coating do not require activation of any sort, hence the coated parts can be stored securely and used immediately with no further treatment.

A number of techniques can be used for coating application. Conventional air pressure atomising nozzles often used for spray coating [34-36]. However, air atomising nozzles should have certain air pressure to ensure atomisation which limits the range of spray velocities that can be controlled. Moreover, droplet size distribution is strongly dependant on the pressure applied, and generally in the region of tens to hundreds of µm which limits the uniformity and thickness control when used over large area. Some companies, such as Sono-Tek, Sonaer, and Optomec are successfully using ultrasonic atomization processes to generate aerosols used for coatings or micro printing. These nozzles however, may require additional mechanism of spray acceleration to optimize the particle velocity at the impact and may suffer from internal condensation/clogging problems.

This manuscript describes a technique of silver nanoparticles deposition using a dual velocity nozzle in combination with the ultrasonic atomizer. That method allows fine droplet velocity and coating thickness control, eliminates clogging and condensation problems, and allows for uniform coating over a relatively large area. The atomizer produces a fine mist of droplets measuring approximately 3um-7um in diameter. Each

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individual droplet contains a small number of nanoparticles from the suspension that has been atomized. When deposited onto the substrate (in this case glass), the liquid rapidly evaporates leaving a thin, uniformly dispersed layer of silver nanoparticles. Furthermore, to the best of our knowledge, silver nanoparticles have not been used to produce superhydrophilic/ antifogging coatings described in this manuscript. Antireflective properties of the obtained coating can also contribute to its usefulness in the applications where increased light transmission is desirable.

Materials 3.2

Spherical and mono-dispersed silver nanoparticles were synthesized by the polyol process [66, 67]. Materials used during the nanoparticles synthesis are silver nitrate (≥99%), ethylene glycol (anhydrous, 99.8%), and polyvinylpyrrolidone (PVP) (Mw ~29000) from Sigma-Aldrich. Initially, 4.5g of PVP was dissolved in 50 ml of ethylene glycol. The solution was stirred vigorously and heated up to 160°C in a silicone oil bath. Subsequently, silver nitrate aqueous solution (2.5g silver nitrate dissolved in 50 ml ethylene glycol) was drop-wisely added to the PVP solution. The mixture was maintained for 30 min at 160°C, with subsequent cooling to room temperature (~23°C). Silver nanoparticles were separated from the solution by centrifugation and washed in ethanol six times. Finally, silver nano-particles were dispersed in D.I. water to use in the coating process.

Transmission electron microscopy (TEM) imaging was taken using JEOL JEM-1400 transmission electron microscope. X-ray diffraction (XRD) measurements were performed on a Rigaku Miniflex diffractometer with a chromium source (kR radiation, λ = 2.2890 Å). Figure 1 shows the TEM images of silver nanoparticles with sizes ranging

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from ~5 nm to 20 nm in diameter and illustrates size uniformity of monodispersed nanoparticles.

Figure 3-1. TEM images of synthesized Ag particles.

XRD analysis shown in Figure 3-2 confirms silver nanoparticles crystal structure (face centre cubic with Fm3m symmetry) with average crystallite size of 14.8 nm. The crystallite grain sizes of the silver nano-particles are calculated by the Debby-Scherer’s equation [68]:

𝑑 = 0.9 𝜆 𝛽 cos 𝜃

where d is the mean crystalline size, λ is the X-ray wavelength, β is the full width at half maximum (FWHM), and θ is the diffraction angle.

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Figure 3-2. XRD analysis results.

Dual Regime Spray System for Coating 3.3

The device used to create the coating consists of an ultrasonic atomizer (2.4 MHz oscillation frequency), dual velocity deposition device developed in-house, and CNC router (Romaxx CNC Systems). The nozzle was mounted onto the router to ensure consistent coating application with set overlap, feed rate, and distance from the nozzle tip to the surface to be coated. A schematic image of the coating system is shown in Figure 3.

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Figure 3-3. Deposition device schematics.

Silver nanoparticles suspended in water are atomized by the ultrasonic atomizer that produces a mist of water droplets with the average diameter of approximately 5 µm. Nanoparticles entrapped within the atomized droplets are carried to the deposition device by carrier gas stream (air in this case) at velocities of approximately 5 – 7 m/s. The 3D model of the deposition nozzle is shown in Figure 3-4.

Atomization based dual regime spray coating system: design and applications

Supervisory Committee

Dissertation Summary

Table of Contents

List of Tables

List of Figures

List of symbols

Introduction

Chapter 1

Design and application of nanoparticle coating system with

Chapter 2

decoupled spray generation and deposition control

Uniform silver nanoparticles coating using dual regime

Chapter 3

spray deposition system for superhydrophilic and antifogging

applications